cogl/cogl-bitmap-conversion.c: Include missed header

The cogl-texture-private.h needs to be included as
_cogl_texture_needs_premult_conversion, so that we can avoid implicit
declaration warnings of that symbol (aka C4013 on MSVC).

Reviewed-by: Neil Roberts <neil@linux.intel.com>
cogl-path: Add a symbols file

This is used for exporting symbols when Cogl-Path becomes a DLL (.so)
of its own, so one can use and process this file with a preprocessor for
e.g. and generate a .def file that can be used for this purpose.  Dist the
symbols file as well.

Reviewed-by: Neil Roberts <neil@linux.intel.com>